<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <button>关灯</button>
    <script>
      var btn = document.querySelector('button')
      var html = document.querySelector('html')
      btn.onclick = function () {
        if (btn.innerHTML == '关灯') {
          html.style.backgroundColor = '#000'
          btn.innerHTML = '开灯'
        } else {
          html.style.backgroundColor = '#fff'
          btn.innerHTML = '关灯'
        }
      }
    </script>
  </body>
</html>
